System for launching a lifeboat

ABSTRACT

A system for launching a lifeboat (13) from davits (14) on an off-shore drilling platform (10) comprises a submerged buoyancy device (16) tethered to the bow of the lifeboat (13) by a cable (17) which passes between the device (16) and the lifeboat. The cable (17) is retained by an inverted hook (18) and passes around pulleys (20 and 22) and is anchored to the sea bed (at 23). In use, as the lifeboat descends, a ring (26) attached to the cable (17) releases same from hook (18) causing buoyancy device (16) to ascend rapidly to the surface thus towing the lifeboat away from the platform (10) to a position (13&#39;) some 100 meters away from the platform.

THIS INVENTION concerns a system for launching a lifeboat from a fixedstructure such as an offshore drilling platform.

The system is intended to overcome the problems encountered whenlaunching lifeboats on the weather or windward side of a drillingplatform where there is a tendency for the lifeboat to be driven intothe legs and pipework of the platform thus preventing rapid escape, forexample in the case of fire and when burning debris may fall into thesea from a position directly above the boat.

It is intended that the system should enable the boat to be drawn a safedistance away from the platform once it is afloat.

According to the present invention, a system for launching a lifeboatfrom a fixed structure such as an off-shore drilling platform,comprising a submerged buoyancy device tethered to the bow of thelifeboat by means of a cable which passes, from the lifeboat to thebuoyancy device, and is retained by a release mechanism fixed near thebase of the structure, and by a submerged guide member fixed at asimilar level but at a distance off the structure, the release mechanismbeing adapted to release the cable as the lifeboat descends from thestructure such that consequent rapid ascent of the buoyancy device drawsthe cable around the guide member thus drawing the lifeboat bow in adirection away from the structure during descent, and towing thelifeboat rapidly away from the structure, once it is afloat.

The invention will now be described further, by way of example only,with reference to the accompanying drawing which illustratesschematically a fixed drilling platform with a lifeboat launching systemembodying the invention.

A drilling platform 10 is supported on legs 11 above the sea bed 12. Anumber of lifeboats, one of which is illustrated at 13, are suspendedaround the platform 10 on davits 14. Access to the lifeboats is via thedeck 15 forming part of the platform 10.

The launching system for each lifeboat comprises a buoyancy device 16which is tethered by a cable 17 to the bow of the lifeboat 13.

The cable 17 passes vertically downwards from the lifeboat to a releasemechanism in the form of an inverted hook 18 anchored to the sea bed at19 near the base of the platform, or to one of the legs 11. At thispoint, the cable carries a fixed ring 26 which locates under hook 18.From there the cable passes horizontally over the sea bed to a secondretaining member in the form of a pulley 20 anchored to the sea bed at21, a distance off the platform. The cable then passes around a secondpulley 22 fixed to the base of the buoyancy device 16 and is anchoredbeyond at 23 to the sea bed.

A marker buoy 24 is attached by a line 25 to the buoyancy device 16.

Hook 18, preferably made of stainless steel, comprises an inclined armdisposed at an angle of 45° or more with respect to a vertical stem, thelatter being some 3 meters in height. A snatch weight 27 is suspendedfrom a pulley 28 which rides on cable 17 between ring 26 and pulley 20.

Location of the buoyancy device 16 close to the sea bed, together withthe low level horizontal run of cable 17 between hook 18 and pulley 20ensures that there are no obstructions in the water, for craftapproaching the drilling platform, save for the marker buoy 24 which isvisible above the surface.

When it is required to launch the lifeboat, it is lowered in the usualmanner from the davits 14 towards the surface. As the lifeboat descends,the cable above ring 26 becomes slack so that the ring 26 is releasedfrom hook 18 thus releasing the cable 17 which enables the buoyancydevice 16 to rise sharply to draw the bow of lifeboat 13 in a directionaway from the drilling platform. The arrangement of pulleys 20 and 22provides a 2:1 ratio of descent of the lifeboat against ascent of thebuoyancy device.

When the buoyancy device reaches the surface as indicated at 16' thelifeboat will have assumed a position as indicated at 13' distant fromthe drilling platform with the cable assuming the configuration asindicated at 17'.

The snatch weight 27 prevents a sudden snatch of the cable and ridestowards pulley 20 as the buoyancy device rises thus ensuring a smoothoperation of the system.

The distance of anchors 21 and 23 along the sea bed from the drillingplatform may be 500 meters where the platform itself is some 150 metersabove the sea bed and the depth of the water is in the region of 120meters. The system causes the cable 17 to tow the lifeboat away from theplatform by a distance of approximately 100 meters i.e. a safe distancefrom the platform.

Action of the snatch weight 27 may be enhanced or replaced by acompressible air bag on the buoyancy device which increases its buoyancya the device ascends. The inherent buoyancy of the device 16 may beselected according to the depth of water and the distance off theplatform, but is preferably in the region of 250 kilograms.

It will be appreciated that the lifeboat is caused to turn away from theplatform as it descends onto the water, which is important in caseswhere it is released on the windward side. Once towed away from theplatform the lifeboat will ride according to the weather on the end ofcable 17. Since the cable will be attached to the lifeboat by a quickrelease shackle, may be released permitting the lifeboat to motorfurther away from the platform.

Preferably, the end of the cable to which the lifeboat is attached isprovided with means which will not pass around guide pulley 20 so thatthe whole system may be recovered and reset for further use.

Conveniently, prior to use there will be means adjacent the lifeboat toindicate a continuing tension in cable 17 so that operators may checkthe serviceability of the system at all times. Furthermore, the locationof marker buoys 24, one for each system, around the platform willprovide a ready indication that all systems are properly set up andready for use.

It is not intended to limit the invention to the above details. Forexample, there may be a 1:1 ratio of the cable around pulley 20 with thebuoyancy device 16 merely attached to a free end of the cable.Alternatively, and in accordance with the depth of water available theremay be a greater than 2:1 ratio by providing further pulleys.

The entire system will be constructed of non-corrodible materials.

The invention is not limited to the example described herein. Forexample, hook 18 and ring 26 may be replaced by a pulley with a deviceattached to cable 17 just above the pulley to release the cabletherefrom as the lifeboat descends towards the water.

I claim:
 1. A system for launching a lifeboat from a fixed basestructure such as an off-short drilling platform, comprising a submergedbuoyancy device tethered to the bow of the lifeboat by means of a towingcable which passes from the lifeboat to the submerged buoyancy device, arelease mechanism fixed with respect to the base structure and retainingsaid towing cable before launch, and a submerged guide member fixed at adistance off the base structure, said towing cable passing around saidguide member between the lifeboat and the buoyancy device, the releasemechanism being arranged to release the towing cable completelytherefrom as the lifeboat descends from the base structure such that noconnection remains between the base structure and the lifeboat andbetween the base structure and the buoyancy device, and such thatconsequent free ascent of the buoyancy device draws the towing cablearound the guide member and draws the lifeboat bow in a direction awayfrom the structure during descent, such that the lifeboat is towedrapidly away from the base structure, once it is afloat.
 2. A systemaccording to claim 1, wherein the structure is an off-short drillingplatform and wherein said cable passes between said release mechanismand said submerged guide member, horizontally over the sea bed.
 3. Asystem according to claim 1, wherein the cable passes, after thesubmerged guide member, around a further guide member fixed to the baseof the buoyancy device and is anchored beyond same substantially at thesame level as the submerged guide member.
 4. A system according to claim3, wherein said submerged guide member and said further guide member arepulleys and serve to provide a 2:1 ratio of descent of the lifeboatagainst ascent of the buoyancy device.
 5. A system according to claim 1,wherein said release mechanism is an inverted hook fixed with respect tothe base structure, and adapted to retain said cable, and includingmeans for releasing the cable from the hook in the form of a ringattached locally to the cable and releasable from the hook when thecable above the latter becomes slack.
 6. A system according to claim 1,including a marker buoy attached by a line to the buoyancy device.
 7. Asystem according to claim 1, including a snatch weight suspended from,and adapted to ride upon, the cable between the release mechanism andthe submerged guide member.
 8. A system according to claim 1, whereinthe fixed structure is an off-shore drilling platform, and the submergedguide member is located some 500 meters along the sea bed from the baseof the platform where the platform itself is some 150 meters above thesea bed and the depth of water is in the region of 120 meters.
 9. Asystem according to claim 1, in which the cable is of sufficient lengththat the lifeboat may be towed away from the fixed structure by adistance of approximately 100 meters.
 10. A system according to claim 1,wherein the inherent buoyancy of said buoyancy device is in the regionof 250 kilograms.
 11. A system according to claim 1, including means toindicate a maintained tension in the cable whereby operators may checkthe serviceability of the system at all times.
 12. A system according toclaim 1, wherein there is a 1:1 ratio of the cable around the submergedguide member with the buoyancy device attached to a free end of thecable.